When it comes to the anatomy of bats, the most fascinating part is undoubtedly their wings.
Made up of a thin membrane of skin stretched over a network of bones, cartilage, and muscles, wings are incredibly strong and flexible, allowing these animals to fly with amazing agility.
The structure of the bones in a bat’s wing is quite interesting. For one, they much shorter and lighter than those of other flying animals, such as birds.
One of the most unique things about these mammals is that, not only can they fly, but bats are also capable of gliding.
When bats are flying, they use their powerful muscles to flap their wings up and down, propelling themselves through the air.
However, when they want to save energy, they can simply spread their wings out and glide on the currents of air. This ability to change between powered flight and gliding is what makes bats such agile flyers.
To learn more about the anatomy of a bat’s wings as well as the different bones found within these creatures, check out the information below:
Understanding Bat Anatomy
The German term for bats is “Fledermäuse,” which means “flying mice,” and it’s easy to see why: many bat species resemble flying rodents.
However, bats are not rodents; they are actually more closely related to primates like monkeys, apes, and humans. In fact, bats are the only mammals capable of true flight.
Although bats have all the basic anatomical structures associated with mammals in general, the fact that they fly has resulted in many of these structures being modified to meet the needs of flight.
For example, bats have a very different skeletal structure than other mammals.
When you consider that there are about 1,000+ species of bats – it is not surprising to find that there is considerable room for variation within the group.
While all bats have wings, the shape and size of these wings can differ significantly from one species to another.
A bat’s body is compromised of four main sections: the head, torso, legs, and wings. The head houses the brain, eyes, ears, and mouth.
The torso contains the lungs, heart, and other internal organs. The legs are used for walking, climbing, and hanging upside down. The wings are used for flying.
The Bat Skeleton
Many bat species are nocturnal, meaning they are most active at night. Their natural history and physiology have been shaped by this nocturnal lifestyle. For example, bats have very good night vision and use echolocation to navigate in the dark.
On the other hand, the bat skeleton is specially adapted for flight. The bones are lightweight yet strong enough to support the body in the air. Bat flight is amazing to watch, and it’s even more amazing when you understand the mechanics behind it.
The Changes Seen in Bat Anatomy
As mentioned above, bats, due to their need for flight, have slowly changed and adapted their skeletal system to be more efficient in the air.
The most obvious changes are of course is the greatly elongated bones of the forelimbs, particularly the metacarpals and phalanges. These finger bones give the wing its shape and provide a large surface area for the attachment of flight muscles.
Looking closely, we will find that some bats have developed an extra bone on the hind limbs near the ankle.
This small bone, which helps support the uropatagium (a flap of skin that extends between the back legs and the tail) is called the calcar, or calcaneum. This structure is not found in all bat species, but it is present in many of them.
The calcar helps to stiffen the wing and gives the bat more control over its flight. It also serves as a point of attachment for some of the muscles that are responsible for keeping the wing extended
Bats also have an extra bone at the elbow. This very small bone, which is the upper arm equivalent of the patella, is called the ulna sesamoid.
A third major change, which is not visible in the diagram, is that the legs of bats are rotated through 180º – which means that their knees flex in the opposite direction to those of a human or a cat.
The Various Bat Bones
In general, bats have:
- 7 cervical (neck) vertebrae;
- 11 thoracic (chest) vertebrae;
- 4 lumbar (abdominal) vertebrae
- Between 0 and 10 caudal (tail) vertebrae.
In a bat, you can expect to find up to 45 bones. Unlike the bones in bird wings that are hollow, bats have flat bones as other mammals do.
As such, if you’re wondering “are bat bones hollow?” The answer is no.
In some bat species, the last cervical and first thoracic vertebrae are fused. The pelvic girdle bones (ilium, ischium, and pubis) are more strongly fused than in other mammals. The Megachiroptera (Old World fruit bats) lack caudal vertebrae and thus have no tail.
The Upper Part of a Bat
The shape of the bat skull varies greatly. This variation is dependent on the animal’s diet, with nectar-feeding bats having long, thin skulls and many insectivorous species having short, blunt skulls.
The bones of a bat’s forelimbs all elongate to some extent, with the degree of elongation increasing as the bones move away from the body. The bones of the thumb (metatarsal 1 and phalange 1), the only digit with free movement, are not greatly enlarged.
The ulna is greatly reduced and frequently fused to the radius, which is strong enough to support the wings. Because the wrist of a bat is so flexible, the wing can be folded down like an umbrella.
In most bats, only the thumb retains a claw, but in some flying foxes, such as the grey-headed flying fox, the 2nd digit also has a small claw.
The Lower Part of a Bat
While the wings and upper limbs of a bat are essential for flying, the legs and feet are not used for this purpose. Still, it’s an important part of the bat’s anatomy as it helps the animal to move around on the ground, climb trees, and even hang upside down.
The hindlimbs of a bat are rotated 180º. Thus, when a bat walks on the ground, its knees stick up into the air. This is necessary to keep the wings from dragging on the ground when they are not being used for flight.
The lower section of the hindlimb is composed almost entirely of the tibia. The fibula is vestigial (like the ulna in the forelimb, only more so) and fused to the tibia. The whole limb can rotate through a wide angle, allowing a hanging bat to swivel its body through a complete circle.
The toes of the hindlimbs all have strong, laterally compressed claws and an automatic locking system involving a tendon that passes through a sheath of cartilaginous rings attached to the phalange, which constrains its movement.
Bats Sleeping – How it Works
A bat’s anatomy plays a huge part in how it is able to fly as well as hang upside down.
When a bat hangs, its legs and feet grip the surface firmly while the tendons in its joints lock the bones into place.
This tendon is so attached that it is the bat’s own weight that keeps it taut. This allows the bat to sleep without falling from its roost.
Birds also have a locking mechanism on their claws to stop them from falling off their perches, but their system is quite different.
When a bat sleeps, it hangs by its hindlimbs and folds its wings around its body. The bones and muscles of the wing are so arranged that the wing is extended/opened and closed through the operation of only a single muscle for each action.
Even when the bat is at rest, this muscle is kept slightly contracted to keep the wing from fully opening.
The shape of the bones is such that lifting – or relaxing – the humerus stretches a muscle attached to the radius, pulling it out or in.
Moving the radius has a similar effect on the carpals and metacarpals (see diagram), thus the whole arm can be opened and closed very quickly and efficiently with a minimum of muscular effort
Perhaps now you’d like to learn about how bats fly.
5 thoughts on “Bat Anatomy 101: The Various Bones of The Wing & Skeleton”
Great information! I had some bones delivered last night to my doorstep.
Most likely by a cat.
I’ve tried to assemble them based on your drawings. Can I send you a photo?
I am not an expert on bat taxonomy, just a general zoologist, my personal research has all been in Europe, on Bird Migration and Insect biodiversity, so there would not be any point sending me a photo. You should contact your local bat group, or your nearest natural history museum.
P.S. Thinking about it bit longer, I’m guessing now that Seba has confused the fibula with the heel bone (calcar), which is especially long and well-developed in the Bulldog Bat. It is misplaced, however, in the illustration that Seba published along with his description. Am I correct? Cheers, Ted
Hi Ted, I don’t have that paper and a google search didn’t find it so I can’t be a lot of help. With over 1300 species of bats there is of course a lot of room for variation and individual deviation from the generalized information I put on a site like this. You are probably correct. Thanks for writing.
Hello Gordon, I hope you can help explain the anatomy of the leg of the Greater Bulldog Bat. Albertus Seba, (1734: 89-90), in describing what is now Noctilio leporinus, says “the hind legs are split between the bones of the tibia and fibula, from the joint of the hip [he must mean to say “knee”] to the heel, keeping a perfect mutual distance, without intermediate body, so that each of these bones is clothed with its own particular covering.” But you say the fibula is vestigial and fused to the tibia. Is the Greater Bulldog Bat perhaps an exception to the latter? Thanks, Ted in Seattle